High-performance gas separation membranes are attractive for molecular-level separations in industrial-scale chemical, energy, and environmental processes. Membrane-separation technology has become a promising alternative to conventional energy-intensive separation processes such as distillation or absorption, for example, in natural gas sweetening, hydrogen recovery and production, carbon dioxide separation from flue gas, and air separation. Molecular sieving materials are widely regarded as next-generation membranes with the capability of simultaneously achieving high permeability and selectivity. Over the past decade, significant progress has been made in devising new types of molecular sieving materials, including zeolite, silica, metal organic frameworks (MOFs), and carbon-based membranes. Polymer membranes, in particular, are capable of providing a more energy-efficient method of gas separation because they do not require thermal regeneration, a phase change, or active moving parts in their operation; therefore, they are expected to play a growing role in an energy-constrained and low-carbon future.
However, there are several aspects of conventional polymer membranes that need improvement before polymer membranes can cost effectively be used on an industrial scale. In particular, most commercial polymer membranes for gas separation are based on a few polymers with low permeability and high selectivity, so they require large areas to compensate for lack of permeance; this increases costs and space requirements for large-scale applications. Microporous polymers with high permeability generally exhibit insufficient selectivity for practical applications, because they possess ill-defined voids that, because of chain flexibility, fluctuate in size, and therefore, have limited size-selectivity. Moreover, the fact that most microporous polymers are generally in a powdered state and insoluble in solvents makes the adaptation of this methodology to the preparation of membranes extremely difficult. Difficulties in processing the micropores into membranes and functionalizing them have limited their controlling gas-separation properties and further hindered the development of this field. Furthermore, membranes based on such microporous materials have not found commercial applications in gas separation because of scale-up impracticalities and high cost. Therefore, there remains the challenge of not only producing improved porous membranes for gas separation, but also, an improved methodology for preparing porous polymeric membranes that provide both the permeability and selectivity needed to support large-scale gas separations.